SHARKS. 105 



teeth ai'e small and generally arranged in several series j the fins 

 are not armed with spines, a nictitating membrane is absent, and 

 the spiracle is always distinct. Two species of Scy Ilium are found 

 on the British coast — the " Larger " and " Lesser Spotted Dog- 

 fish," S. caniculum and S. catulus. They are coast-fishes, living 

 on the bottom, and feeding on crustaceans, dead fishes, &c. 

 The Zebra- or Tiger- Shark, Stegostoma tigrinum, one of the 

 commonest and handsomest Sharks of the Indian Ocean, reaches 

 a size of 10 to 15 feet. Crossorhinus (Case 39), a genus of ground 

 Sharks from the Indo-Pacific Ocean ; they are sluggish and lie 

 concealed on the bottom watching for their prey. In accordance 

 with this habit their colour closely assimilates that of a rock or stone 

 covered with short vegetable and coralline growth, a resemblance 

 which is increased by the frond-like tentacles on the side of the 

 head. 



The Cestraciontidce (Case 40) have, like the preceding, several [Case 40. J 

 series of teeth simultaneously in function ; each of the dorsal fins 

 is armed with a spine. This family is one of particular interest, 

 because representatives of it occur in numerous modifications in 

 Primary and Secondary strata. Their dentition (of which pre- 

 parations are shown in Table-case C) is adapted for the prehension 

 as well as mastication of crustaceous and hard-shelled animals. A 

 few recent species are known, from various parts of the Pacific 

 Ocean. 



The Spinacidce, or Spiny Dog-fishes (Cases 39, 40), so called on 

 account of the spine with which their dorsal fins are usually armed, 

 are a family containing numerous genera, the majority of the species 

 being of small size. Acanthias is the best known, the two species, 

 A. vulgaris and A. blainvillii, occurring on the British coast. The 

 species of Centrophorus live at a considerable depth, perhaps at a 

 greater depth than any of the other known Sharks. The Portuguese 

 fishermen fish for them in 400 or 500 fathoms with a line of some 600 

 fathoms in length. The Greenland Shark (Lcemargus bo?'ealis) is an 

 inhabitant of the Arctic regions, rarely straying to the latitudes of 

 Great Britain. It grows to a length of about 15 feet, and is extremely 

 voracious. The " Spinous Shark " (Echinorhinus spinoms) is a 

 ground Shark, probably living at some depth, and but accidentally 

 coming to the surface. More frequently met with in the Medi- 



